Windows Client: Broken files when downloading from NC Server

Files are broken when after downloading from a NC Server after syncing.
After syncing there is e.g. a file named .IMG_20210223_130701.jpg.~1b19 an the disc. If I manually remove the part .~1b19 from the file I can open the image, so the file itself is not damaged.

Syncing worked perfectly like a charm and from one day to another I noticed this bug so I cannot define, what I have changed so that it ends in this result.

The failture is on a up-to-date Windows 10 PC with NC-Client 3.1.1. Server is also up-to-date NC 20.8. Can´t find warnings in th log. I reinstalled the client on the W10 PC and synced from scretch download ing all files from the server getting the result, that all files have had the wrong extention.

There are 3 Windows 10 PCs and 2 Android deviced connected to this NC-Server. On each device syncing (up- and downloading) is working well without destroing a file (-name), just this specific Windows PC.

Does anyone have a idea what happens on this specific Windows PC?

In advance thanx for each help!